Protein-binding of secretin in human plasma

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Abstract
Protein-binding of endogenous plasma secretin and of 125I-labeled secretin incubated with charcoal-treated plasma examined by gel filtration on a Sephacryl S-200 Superfine column (16 .times. 980 mm) showed that secretin in plasma appears both to be bound to at least 2 different plasma proteins where albumin appears to be the major binding protein, and also to occur as a free molecular form. Protein-binding studied by incubating 125I-labeled secretin with charcoal-treated plasma under various conditions followed by charcoal separation of bound from free label indicated the presence of more specific secretin-binding sites on the plasma proteins with an avidity comparable to that otherwise reported for albumin as a binding protein. The protein-binding of 125I-labeled secretin was optimal or reached equilibrium after 2 days incubation at 20.degree. C and first after 8 days incubation at 4.degree. C. Also, the protein-binding of 125I-labeled secretin was higher at an incubation temperature of 20.degree. than at 4.degree. C; was optimal at pH 7.4; increased with increasing amounts of charcoal-treated plasma up to an amount of 800 .mu.l in this assay system before leveling off; and increased in a constant and predictable manner with increasing amounts of 125I-labeled secretin at least with the amounts of labeled secretin examined here.

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